Double null liquid metal diverters

ABSTRACT

A tokamak plasma vessel. The tokamak plasma vessel comprises a toroidal plasma chamber, a plurality of poloidal field coils, an upper divertor assembly, and a lower divertor assembly. The plurality of poloidal field coils are configured to provide a poloidal magnetic field having a substantially symmetric plasma core and an upper and lower null, such that ions in a scrape off layer outside the plasma core are directed by the magnetic field past one of the upper and lower nulls to divertor surfaces of the respective upper and lower divertor assembly. Each of the upper and lower divertor assembly comprises a liquid metal inlet and a liquid metal outlet located below the liquid metal inlet. Each of the upper and lower divertor assembly is configured such that in use liquid metal flows from the liquid metal inlet to the liquid metal outlet over at least one divertor surface of the divertor assembly.

CROSS-REFERENCE TO RELATED APPLICATIONS

The application is a continuation application of U.S. patent applicationSer. No. 17/253,794 filed Dec. 18, 2020, which is a U.S. national stageentry of International Patent Application No PCT/GB2019/051760, filed onJun. 21, 2019, which claims the benefit from British Application No.1810512.2, filed Jun. 27, 2018, all of which are incorporated byreference.

FIELD OF THE INVENTION

The invention relates to tokamak plasma chambers, in particular to thedivertors of tokamak plasma chambers.

BACKGROUND

A divertor is a device within a tokamak plasma vessel which allows forremoval of waste material and power from the plasma while the tokamak isoperating. The waste material naturally arises as particles diffuse outfrom the magnetically confined plasma core. The waste particles are acombination of the fuel (Deuterium and Tritium), fusion products (heliumash), and heavier ions released from the walls. To confine the plasma,tokamaks utilise magnetic fields. However, particles slowly and randomlydiffuse out, and eventually impact one of the divertor surfaces, whichare configured to withstand the high flux of ions.

A poloidal cross section through one side of an exemplary tokamak isillustrated in FIG. 1. The tokamak 100 comprises a toroidal plasmachamber 101. Poloidal magnetic field coils produce a poloidal magneticfield to confine the plasma (which is carrying a current). If there wereno collisions between plasma particles, turbulence, waves or other suchphenomena, then the plasma (made from charged particles) wouldeffectively be tied to magnetic field lines (which can be represented aslines of constant poloidal flux 113). The plasma is said to be confinedonto lines of constant poloidal flux inside the “plasma core” becausethe lines of constant flux are closed, so called “closed flux surfaces”.Through collisions and other such processes, the plasma slowly diffusesout of the plasma core. The “last closed flux surface” 111 has a nullpoint 112 at one end (usually the lower end) and defines the edge of theconfined core. Flux lines 114 immediately outside the plasma core (the“scrape off layer”) intersect two surfaces below the null: the outboard(i.e. radially outer) divertor surface 121 (located in this example atthe bottom of a channel in the lower part of the plasma chamber), andthe inboard (i.e. radially inner) divertor surface 122. Waste particlesand power are deposited onto these surfaces, with the majority of thewaste particles and power landing on the outboard divertor surface (theexact split between inboard and outboard depends on turbulent physicswithin the scrape off layer). The divertor surfaces are constructed fromelements having relatively low atomic numbers (to avoid contaminatingthe plasma with high atomic number ions through sputtering and othersuch erosion processes) which are metals (to avoid tritium retentionwithin the divertors). Suitable metals include tungsten, molybdenum,beryllium, lead-lithium or lithium.

A cross section through a second exemplary tokamak is illustrated inFIG. 2. This tokamak has a “double null” divertor. The principle of the“double null” divertor is the same as for the “single null” divertor inFIG. 1, except that nulls 211, 212 and corresponding divertor surfaces221, 222, 223, 224 are provided at both the upper and lower edges of theplasma core 210. An advantage of the double null configuration is thatthe heat flux on each divertor surface is roughly half that which wouldbe experienced by a single null configuration.

SUMMARY

According to an aspect of the present invention, there is provided atokamak plasma vessel. The tokamak plasma vessel comprises a toroidalplasma chamber, a plurality of poloidal field coils, an upper divertorassembly, and a lower divertor assembly. The plurality of poloidal fieldcoils are configured to provide a poloidal magnetic field having asubstantially symmetric plasma core and an upper and lower null, suchthat ions in a scrape off layer outside the plasma core are directed bythe magnetic field past one of the upper and lower nulls to divertorsurfaces of the respective upper and lower divertor assembly. Each ofthe upper and lower divertor assembly comprises a liquid metal inlet anda liquid metal outlet located below the liquid metal inlet. Each of theupper and lower divertor assembly is configured such that in use liquidmetal flows from the liquid metal inlet to the liquid metal outlet overat least one divertor surface of the divertor assembly.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section of a tokamak having a single-null divertor;

FIG. 2 is a cross section of a tokamak having a double-null divertor;

FIG. 3 is a cross section of a tokamak having liquid metal divertorsurfaces;

FIG. 4 is a schematic illustration of a liquid metal divertor surface;

FIG. 5 is a cross section of an alternative tokamak having liquid metaloutboard divertor surfaces and solid inboard divertor surfaces;

FIG. 6 is an exemplary illustration of the possible angles for a liquiddivertor surface;

FIG. 7 is a cross section of an alternative tokamak having liquid metaldivertor surfaces;

FIG. 8 is a cross section of a divertor assembly of a tokamak havingliquid metal divertor surfaces;

FIG. 9 is a schematic illustration of a liquid metal circulation system.

DETAILED DESCRIPTION

A flowing liquid metal divertor surface can be provided by using metalswhich are liquid at the temperatures within the plasma vessel. Such asurface can quickly recover from transient high-heat flux events in theplasma (e.g. Edge Localised Modes, ELMs). However, flowing liquid metaldivertors are difficult to provide in a double null configuration, asthe liquid metal must either be provided on an upward facing surface ofthe divertor (whereas, for prior art double null divertors, the divertorsurface for the upper divertors would be downward facing), or held on toa downward facing surface by non-gravitational means which are difficultto provide and may result in uneven flows due to “negative pressure” inthe liquid metal—effectively, the liquid metal must flow “on theceiling”.

A double null divertor using flowing liquid metal can be provided if theupper divertor surfaces are not symmetrical to the lower divertorsurfaces. This can be achieved either with a symmetric magnetic field,or a non-symmetric magnetic field. Using a non-symmetric magnetic fieldgives more design freedom to the positioning of the divertor surfaces,but adds additional complexity to the poloidal field coils, due to theneed to provide the non-symmetric field outside the plasma core whilestill keeping the plasma core substantially symmetrical. In contrast,using a symmetric magnetic field more tightly constrains the divertorsurface positions, but simplifies the design of the poloidal fieldcoils.

FIG. 3 shows a cross section of an exemplary tokamak 300 having anon-symmetric magnetic field and a double null divertor with alldivertor surfaces being flowing liquid metal. The tokamak 300 comprisesa plasma vessel having a central column 302. The tokamak comprisespoloidal field coils configured to provide a magnetic field such thatthe plasma core itself 311 is substantially symmetric, and such that thescrape off layer 312 is directed to each of the divertor surfaces. Thedivertor surfaces are the upper inboard divertor surface 321, the upperoutboard divertor surface 322, the lower inboard divertor surface 323,and the lower outboard divertor surface 324. Each divertor surface hasthe structure shown in FIG. 4, and comprises a structural support (withcooling channels inside) 401 over which flows a liquid metal layer 402.The liquid metal layer is provided from an inlet 403, and drains into anoutlet 404, with the inlet being located above the outlet such that theliquid metal flows under gravity. Further support structures 303 areprovided to keep the divertor surfaces in place. The outboard divertorsurfaces (which take the majority of the waste particles and power) arearranged such that they intersect the scrape off layer 312 at a shallowangle (e.g. angles of 2 degrees are widely quoted within the literature;this is set by engineering tolerances). The tokamak also includespassive stabilisation plates 304 which act to reduce the growth rate ofvarious plasma instabilities (necessary so that the active controlsystem can control the plasma). The optimum position of the passiveplates is near the null point and so should be considered in the designof the divertor system despite performing a different function.

The angle of the liquid metal flow can range from steep to shallow. Infact, it is possible for the divertor surface to be inverted, such thatit faces partially downwards. The liquid metal will flow on theunderside of a surface provided that the wetting angle and surfacetension of the liquid metal can counteract the gravitational force. Thiswill depend on the angle between the surface and the vertical. Therewill be a critical angle for a given combination of surface, liquidmetal, and flow rate above which the metal will not be able to flow onthe underside (this angle may be up to 90°, at which point the liquidmetal will wet even a horizontal downward facing surface). Greaterangles of inversion may be achieved by increasing the wetted area of thesurface (e.g. by altering the geometry of the surface, or providingchannels or additional roughness on the surface), by providing a surfacewhich has an increased wetting angle with the liquid metal, or by usinga thinner liquid metal flow.

Electromagnetic forces will occur within the liquid metal duringoperation of the tokamak, and these can be engineered such that theywill assist in counteracting the effect of gravity on the liquid metal.This may include providing a current through the liquid metal so thatthe interaction of the current and the magnetic field counteractsgravity.

FIG. 6 provides an example of the acceptable angles for an exemplarydivertor at a certain flow rate. The divertor 600 may be oriented at anangular range represented by the arc 601 (with the arc representing thedirection of a line perpendicular to the surface). The segment of arcbetween points B and C is where the surface is “generally upward facing”as the term is used in the examples herein—i.e. the line perpendicularto the surface has a component which extends upwards (even if thiscomponent may be small compared to the horizontal component for a steepincline). The arcs between points A and B, and between points C and Drepresent examples where the surface is “generally downward facing”,with an angle such that the liquid metal flow will remain on the surfacedespite gravity. The arc between A and D (not passing through B or C)represents the examples where the liquid metal will not be able toremain on the surface—divertors at these angles would not be suitable.

The inversion angle achievable (i.e. the angle of the surface comparedto a vertical surface) for a particular configuration of surface andliquid metal may be determined by trial and error. For example, this maybe done creating such a surface, affixing it to a pivot within a vacuumchamber (at similar pressure, temperature, and electromagneticconditions to those expected in use), and flowing liquid metal over thesurface over a range of angles until the liquid metal no longer adheres.Alternatively, the inversion angle achievable may be determined byappropriate simulation as known in the art—i.e. a fluid simulation thattakes into account wetting and magneto-hydrodynamic effects.

The liquid metal may flow in a radially inward direction, i.e. eachinlet 403 may be located radially outwards of the corresponding outlet404. This will cause the surface area of the liquid metal to decreasealong the flow (as the divertor surfaces are substantially annular—bearing in mind that FIG. 3 is a cross section through an object withcylindrical symmetry around the central column 302) this again is moreimportant for the outboard divertor surfaces. In the absence of themagnetic field, the thickness of the liquid would increase along theflow, and this effect will occur to a lesser extent with the magneticfield. This allows the divertor to recover more quickly from a dry out(i.e. the removal of a substantial part of the liquid metal from thedivertor, e.g. due to a transient high heat flux event such as an ELM).As can be seen in FIG. 3, it is particularly difficult to achieve theseoptional features for the inboard divertor surfaces 322 and 324 (indeed,the inboard upper divertor surface 322 has neither feature).

In an alternative construction, shown in FIG. 5, the poloidal magneticfield is symmetric and the inboard divertor surfaces 521, 523 may bemade from solid metal (e.g. tungsten, molybdenum, or any other suitablemetal known in the art), with the other features being the same as forFIG. 3. Although the solid metals are less effective than the liquidmetal divertors, this is of relatively little concern for the inboarddivertors as they experience considerably less heat flux than theoutboard divertors. As a further alternative, only one of the upperinboard divertor surface or the lower inboard divertor surface may bemade from solid metal, with the other divertor surfaces being liquidmetal divertor surfaces as described above.

The use of a symmetric poloidal magnetic field does not require solidinboard divertor surfaces, or vice versa. A symmetric magnetic field maybe used with liquid inboard divertor surfaces, or solid inboard divertorsurfaces may be used with an asymmetric magnetic field.

A further alternative construction is shown in FIG. 7. In this example,lower divertor surfaces 323, 324 have the same structure as in FIG. 3,but the upper divertor surfaces 721, 722 are arranged such that theupper outboard divertor surface is generally downwards facing.

The previous examples have assumed that the entirety of a given divertorregion (e.g. the upper inboard divertor surface, or the lower outboarddivertor surface) is either liquid or solid. However, this need not bethe case. FIG. 8 shows a hybrid divertor (only the lower divertorsurfaces are shown, but the same principle applies to a double nulldivertor). The divertor comprises an inboard strike point divertorsurface 801, and outboard strike point divertor surface 802, an inboardfar divertor surface 803, and outboard far divertor surface 804, and aninboard private divertor surface 805 and outboard private divertorsurface 806.

The strike point divertor surfaces 801, 802 are solid, and are locatedat the “strike points”—the locations where the magnetic flux lines 810corresponding to the null 811 strike the divertor. The far divertorsurfaces 803, 804 are located in the “far-scrape off layer”, i.e.inboard of the inboard strike point divertor surface and outboard of theoutboard strike point divertor surface, respectively. The inboard 805and outboard 806 private divertor surfaces are located in the “privateregion”, i.e. between the inboard and outboard strike point divertorsurfaces.

Any combination (or all) of the private and far divertor surfaces may beliquid metal divertor surfaces, as described previously. Thisarrangement gives a good balance of the resistance of a solid divertorsurface to high heat flux (in case the peak heat flux is sufficientlyhigh to disrupt the liquid flow), in combination with the additionalparticle pumping provided by the liquid metal surfaces (i.e. removingthe scrape-off layer particles from the plasma). This arrangementfunctions because the heat flux on the divertor decays approximatelyexponentially with distance from the strike points—so in the case wherethe peak heat flux at the strike point is excessively high, liquid metaldivertor surfaces may still be used further away from the strike point.

As an alternative, a single private divertor surface may be providedwhich spans between the inboard and outboard strike point divertorsurfaces.

FIG. 9 is a schematic illustration of a liquid metal circulation systemfor use with the liquid metal divertor surfaces. The liquid metaldivertor 910 has an inlet 911 and an outlet 912. The circulation systemcomprises a pump 901 and a reservoir 902 (plus associated conduits andvalves). The reservoir 901 supplies liquid metal to the inlet 911, at aconsistent flow rate, due to gravity. Liquid metal exiting the outlet912 is raised back to the reservoir 901 via the pump 901.

Alternatively, a system which comprises a pump but not a reservoir maybe used (with the pump directly supplying liquid metal from the outlet912 to the inlet 911). In general, any liquid metal supply system whichprovides a consistent flow rate is suitable. In particular, whendesigning downward facing divertor surfaces, the flow rate will in partdetermine the angles at which the downward facing surface can be placed.

The liquid metal supply may be a circulation system as described above,or it may comprises a reservoir which is refilled from an externalsource periodically.

Cleaning and/or filtration means may be provided within the liquid metalcirculation system, to clean any waste products from the liquid metal.Alternatively or additionally, the circulation system may include portsfor removing liquid metal from the circulation system, and replacing itwith liquid metal that does not have the waste products.

Lithium is a preferred metal for liquid metal divertor surfaces as it isthe lowest atomic number element which is suitable (and therefore causesthe least contamination of the plasma). As an alternative, tin or othermetals with suitably low atomic number, may be used. Or a combinationsuch as tin-lithium.

1. A tokamak plasma vessel comprising: a toroidal plasma chamber; aplurality of poloidal field coils; an upper divertor assembly; a lowerdivertor assembly; wherein the plurality of poloidal field coils areconfigured to provide a poloidal magnetic field having a substantiallysymmetric plasma core and an upper and lower null, such that ions in ascrape off layer outside the plasma core are directed by the magneticfield past one of the upper and lower nulls to divertor surfaces of therespective upper and lower divertor assembly; wherein each of the upperand lower divertor assembly comprises: a radially inboard divertorsurface formed from solid meal and positioned to receive ions from thescrape off layer which is radially inward of the plasma envelope; aradially outboard divertor surface positioned to receive ions from thescrape off layer which is radially outwards of the plasma envelope, aliquid metal inlet; and a liquid metal outlet located below the liquidmetal inlet; configured such that in use liquid metal flows from theliquid metal inlet to the liquid metal outlet over at least therespective radially outboard divertor surface.
 2. A tokamak plasmavessel according to claim 1, wherein the poloidal field coils areconfigured to provide a symmetric magnetic field.
 3. A tokamak plasmavessel according to claim 1, wherein the poloidal field coils areconfigured to provide a magnetic field which is asymmetric outside theplasma core so as to optimise interaction with the upward facingdivertor surfaces.
 4. A tokamak plasma vessel according to claim 1, andcomprising a liquid metal supply means configured to supply liquid metalto each liquid metal inlet at a respective flow rate.
 5. A tokamakplasma vessel according to claim 1, wherein each divertor surface overwhich the liquid metal flows is generally upward facing.
 6. A tokamakplasma vessel according to claim 4, wherein the upper divertor assemblycomprises at least one divertor surface over which liquid metal flowswhich is generally downward facing, and wherein that divertor surface isat an angle such that, when the liquid metal supply means suppliesliquid metal to the surface at the respective flow rate, the wetting ofthe liquid metal to the divertor surface prevents liquid metal fromfalling from the divertor surface.
 7. A tokamak plasma vessel accordingto claim 6, wherein the divertor surfaces are arranged with reflectivesymmetry, such that the divertor surfaces of the upper divertor assemblyare a reflection of the divertor surfaces of the lower divertor assemblyin an equatorial plane of the tokamak plasma vessel.
 8. A tokamak plasmavessel according to claim 6, wherein the generally downward facingdivertor surface comprises channels.
 9. A tokamak plasma vesselaccording to claim 1, wherein the liquid metal inlet of at least one ofthe divertor surfaces is located radially outwards of the respectiveliquid metal outlet.
 10. A tokamak plasma vessel according to claim 1,wherein the liquid metal is lithium or tin.